Re: [Qemu-devel] [PATCH] [RFC] Add machine type pc-1.0-qemu-kvm for live migrate compatibility with qemu-kvm

[Alex Bligh]

Andreas,

On 27 Jul 2014, at 15:10, Andreas Färber <address@hidden> wrote:

> Hi Alex,
> 
> + quintela, mst, libvirt

Thanks for your comments!

> This sounds like a really cool feature that SUSE would probably be
> interested in extending back to 0.14 and 0.15, but I see a fundamental
> flaw: libvirt on those old source systems does not know it should use a
> different machine name on the destination side and would still use
> pc-1.0, wouldn't it? After all, it needs to be able to migrate to other
> old qemu-kvm machines, so it can't just be updated to use the new name.

Right. I'm not using libvirt but face a similar problem. The destination
libvirt (or equivalent) now has two 'pc-1.0' machine types. One (allegedly)
use by qemu-git 1.0, and one used by qemu-kvm 1.0. The name 'pc-1.0' needs
to refer to exactly one of these (obviously), and it currently refers
to qemu-git 1.0, so I've left it that way.

In a distribution environment where qemu-1.0 was never used, one might
want to make my new machine type called 'pc-1.0' and the existing qemu.git
pc-1.0 called 'pc-1.0-qemu-git' or similar.

Why don't I just change it? Well, imagine someone running Ubuntu 13.04 (say)
which shipped with an early post merge qemu made a machine with machine type
pc-1.0. Despite the fact that Ubuntu 12.04 was shipped with qemu-kvm (and
hence -m pc-1.0 there meant qemu-kvm's pc-1.0), on 13.04, this would mean
qemu-git's pc-1.0, and changing the name would break the migration of an
Ubuntu 13.04 pc-1.0 machine, whilst fixing migration of a 12.04 pc-1.0
machine. Let no one say this isn't a mess.

So, I'm expect whatever generates the qemu command line to know (from the
other end) whether to use the different version.

Perhaps a slightly nicer fix, if somewhat rococo, would be:
* Use pc-1.0-qemu-kvm as one machine name
* Use pc-1.0-qemu-git as another machine name
* Make pc-1.0 an alias of either one or the other, configurable at the
  command line, and subject to a build-time default.

This would let distributions / users simply decide which kind of pc-1.0
migration they'd like by working by default, and which they'd like
broken; the broken one is a second class citizen which can only
handle migration by changing the qemu command line appropriately.

This would perhaps make things more transparent by default.

BTW I would agree this is a significant issue, having spent quite a lot
of today playing with stuff that (mis)parses "qemu -version"

Also BTW, I did wonder whether I could autodetect this from the stream.
However, as it has to be read serially, by the time you know it's wrong,
you are too late AFAICT. Useful suggestion in case of future accidental
breakage: put the qemu version in a section right up top.

> Minor bikeshedding: I would ask to keep the package name in front of the
> machine version, e.g. qemu-kvm-pc-1.0. Or just kvm-pc-1.0 since this is
> a QEMU parameter anyway.

Obviously not a matter of great import, but per the list
below, the standard appears to be 'pc-(arch)?-version'
and I'd taken this as a subversion.

# qemu-system-x86_64 -machine '?'
Supported machines are:
pc-0.13              Standard PC (i440FX + PIIX, 1996)
pc-i440fx-2.0        Standard PC (i440FX + PIIX, 1996)
pc-1.0               Standard PC (i440FX + PIIX, 1996)
pc-q35-1.7           Standard PC (Q35 + ICH9, 2009)
pc-1.1               Standard PC (i440FX + PIIX, 1996)
q35                  Standard PC (Q35 + ICH9, 2009) (alias of pc-q35-2.0)
pc-q35-2.0           Standard PC (Q35 + ICH9, 2009)
pc-i440fx-1.4        Standard PC (i440FX + PIIX, 1996)
pc-i440fx-1.5        Standard PC (i440FX + PIIX, 1996)
pc-0.14              Standard PC (i440FX + PIIX, 1996)
pc-0.15              Standard PC (i440FX + PIIX, 1996)
xenfv                Xen Fully-virtualized PC
pc-q35-1.4           Standard PC (Q35 + ICH9, 2009)
isapc                ISA-only PC
pc-0.10              Standard PC (i440FX + PIIX, 1996)
pc                   Ubuntu 14.04 PC (i440FX + PIIX, 1996) (alias of 
pc-i440fx-trusty)
pc-i440fx-trusty     Ubuntu 14.04 PC (i440FX + PIIX, 1996) (default)
pc-1.2               Standard PC (i440FX + PIIX, 1996)
pc-0.11              Standard PC (i440FX + PIIX, 1996)
pc-i440fx-1.7        Standard PC (i440FX + PIIX, 1996)
pc-i440fx-1.6        Standard PC (i440FX + PIIX, 1996)
none                 empty machine
xenpv                Xen Para-virtualized PC
pc-q35-1.5           Standard PC (Q35 + ICH9, 2009)
pc-1.0-qemu-kvm      Standard PC (i440FX + PIIX, 1996)
pc-q35-1.6           Standard PC (Q35 + ICH9, 2009)
pc-0.12              Standard PC (i440FX + PIIX, 1996)
pc-1.3               Standard PC (i440FX + PIIX, 1996)

-- 
Alex Bligh